Protective circuit arrangement for a switching transistor in an inductive load circuit

ABSTRACT

A protective circuit arrangement for a switching transistor in an inductive load circuit has a series circuit consisting of a resistor and a controlled auxiliary valve shunted across the collector-emitter path of the switching transistor to take over the load current of the switching transistor when it is switched off. The control electrode of the auxiliary valve is controlled by a control device which consists of a capacitor and a charging resistor in series. A Zener diode may be shunted across the control device. The protective circuit arrangement may be used particularly in an inverter which has a plurality of switching transistors.

Unified States Patel 1 91 Gratzke 1 Apr. 24, 1973 s41 PROTECTIVE cIRcUIT56 References Cited ARRANGEMENT FOR A SWITCHING TRANSISTOR IN ANINDUcTIvI: LOAD UNITED STATES PATENTS CIRCUIT 3,205,412 9/1965 Winston..3l7/DlG. 4 3,287,608 11/1966 P k t ..3l7 DIG. 6 [75] Inventor Wernercratzke Erlangen 3,390,306 6/1968 31 7/33 R many [73] Assignee: SiemensAktiengesellschaft, Munich, Primary Examiner-James D. Trammell GermanyAtt0rney-Hugh A. Chapin [22]- F1led: Aug. 10, 1972 ABSTRACT 27 2 [211App] No 9 39 A protective circuit arrangement for a switching vtransistor in an inductive load circuit has a series cirl l ForeignApplication i y Data cuit consisting of a resistor and a controlledauxiliary May 12 1972 Germany P 22 23 376] valve shunted across thecollector-emitter path of the 7 switching transistor to take over theload current of [52] the switching transistor when it is switched off.The

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control electrode of the auxiliary valve iscontrolled by a controldevice which consists of acapacitor and a charging resistor in series. AZener diode may be shunted across the control device, The protectivecircuit arrangement maybe used particularly in an inverter which has 'aplurality of switching transistors.

' 14 Claims, 4 Drawing Figures PROTECTIVE CIRCUIT ARRANGEMENT FOR ASWITCHING TRANSISTOR IN AN INDUCTIVE LOAD CIRCUIT BACKGROUND OF THEINVENTION 1. Field of the Invention This invention relates to the fieldof transistorized electrical circuitry and, more particularly, to aprotective circuit arrangement for a switching transistor which isarranged in an inductive load circuit for switching the load current onand off.

2. Description of the Prior Art It is customary in engineering practiceto use one or several parallel or series-connected switching transistorsfor switching on and off the load current in an inductive load circuit.It is, for instance, known to connect an inductive load of any desiredkind to a DC source through the switching transistor of a DC switch andto adjust the mean value of the pulsating DC current through the choiceof the on and off duration. It is also known in this art to constructinverters with switching transistors and to connect them to an inductiveload, such as a rotating-field machine. Inverters with bridge circuitsmay contain four, six or more switching transistors.

A switching transistor, which is arranged in a load circuit containinginductances, is subjected to particular stresses if it is switched onand off by application or removal, respectively, of its base current.Particularly in switching off, overvoltages can occur which lead to itsdestruction, as the magnetic energy stored in the inductances of theload circuit to be switched off leads to a high energy density in itsbarrier layer during the switching-off process. If the inductances areparticularly large, and/or if the switching-off process proceedsespecially fast, the maximum permissible energy density may be exceeded,which leads to a so-called second breakdown. For special switchingtransistors of high power rating, a maximum amount of energy E,,, istherefore stated which this switching transistor is still capable ofswitching off. This maximum amount of energy E,,,, is given by E,,, 0.5L 1 where L is the maximum permissible value of the inductance L and themaximum permissible value of the load current 1 In order to keep theenergy density occurring upon switching off in the barrier layer belowthe maximum value permissible for the switching transistor, switchingtransistors are used which are designed with respect to their rating forthe switching-off process and are therefore overdesigned for nominaloperation with the load current on. As compared to the application inwhich a purely resistive load is present in the load circuit, aconsiderably reduced utilization results for the case of a load circuitwhich includes inductances.

It has also been found that if the maximum permissible energy E is to beswitched off in a switching transistor in an inductive load circuit, arelatively long switching-off time must be allowed in order to avoiddangerous overvoltages. This means that the reverse voltage must riseonly slowly during the switching-off process. This is particularlydetrimental in switching transistors for inverters. If the reversevoltage rises slowly, the load current commutates correspondingly slowlyto the next switching transistor. Long commutation times, however, arenot favorable for inverter operation at higher frequencies.

The measures known in the art to eliminate the excessive stress ofswitching transistors, particularly in an inverter with connectedinductive load, have produced only limited success. The protectivemeasures known to those skilled in this art and the problems with suchmeasures include the following:

1. Although a Zener diode shunted across the collector-emitter path ofthe switching transistor protects against over-voltage during switching,it nevertheless brings the switching transistor into the critical regionof second breakdown. Also, because of its low dissipation, the use of aZener diode is confined to low power load circuits.

2. Although capacitor-resistor combinations connected across thecollector-emitter path of the switching transistor permit aswitching-off process in which second breakdown does not occur, thiscombination prolongs the switching-off time considerably. The appearanceof large displacement currents mitigates against operation at higherswitching-off frequencies or inverter operation with higher outputfrequencies, respectively.

3. Overvoltage arresters cannot be used at the low voltages usuallyfound in load circuits with switching transistors because of theircharacteristics.

4. A varistor arranged in shunt to the collectoremitter path of theswitching transistor would lead to high continuous losses. However, theswitching transistor is to be highly utilized voltage-wise.

SUMMARY OF THE INVENTION It is an object of this invention to describe aprotective circuit arrangement for a switching transistor which isarranged for switching on and off the load current in an inductive loadcircuit. The protective circuit arrangement limits the overvoltageoccurring in switching off the load currentto a permissible magnitude,prevents second breakdown and keeps the switching-off losses of theswitching transistor low. The protective circuit arrangement is designedso that it can also be used with switching transistors in an inverter,so that the inverter can be operated at output frequencies higher thanhas been customary up to now.

According to this invention, the foregoing problem in the prior art issolved by a protective circuit arrangement in which a series circuitconsisting of a resistor and a controlled auxiliary valve in the mannerof a transistor is connected parallel to the collector-emitter path ofthe switching transistor and the control electrode of the auxiliaryvalve is controlled by a control device as a function of the voltagerise at the switching transistor.

This protective circuit arrangement can be used with a PNP as well aswith an NPN switching transistor. The resistor connected in series withthe auxiliary valve can be either an ohmic resistance or avoltage-dependent resistance and a PNP or an NPN transistor can be usedas the controlled auxiliary valve. The resistor absorbs the energyreleased in the switching-off process and converts it into heat.Overall, the protective circuit arrangement acts like avoltage-dependent resistance which is connected parallel to thecollector-emitter path of the switching resistor.

In another embodiment of the protective circuit ar-' rangement, thecontrol device consists of a series circuit of a capacitor with acharging resistor, which is connected between the control electrode ofthe auxiliary valve and the collector of the switching transistor. Thecharging resistor can be embodied, in some cases, by the leads of thecapacitor alone. This embodiment can be further developed so that theseries circuit consisting of the capacitor and the charging resistor isshunted by a Zener diode, the Zener voltage of which is chosen so thatthe auxiliary valve is in full conduction if this voltage value isapplied to its control electrode.

The protective circuit arrangement of this invention can be usedadvantageously with all switching transistors which are arranged in aninductive load circuit, such as in a DC selector switch. However, itsadvantages come to the fore particularly if it is used with switchingtransistors of an inverter which is designed with switching transistorsand is connected to a DC source. In principle, one can use a separateprotective circuit which is shunted across each switching transistor ofthe inverter. However, it is less expensive and therefore advisable toprovide a single protective circuit for an entire group of switchingtransistors. In that case the arrangement can be made so that theprotective circuit is connected through a diode poled in the conductiondirection of the auxiliary valve to all those switching transistors ofthe inverter which are connected to the positive input terminal of thelatter. correspondingly, an arrangement can also be provided in whichthe protective circuit is connected through a diode poled in theconduction direction of the auxiliary valve to all those switchingtransistors of the inverter which are connected to the negative inputterminal of the latter. Both arrangements can also be providedsimultaneously side by side.

An advantage of this invention beyond the present state of the art isthat a switching transistor equipped with this protective circuit can beutilized up to its thermal current limit and up to the full reversevoltage. Practical tests have shown that the switching transistor can bestressed more highly by a factor of to with respect to switchingcapacity than without protective circuit. This stated factor of 5 to -10results from a current-carrying capacity higher by a factor of about 4and a higher voltage stress by a factor of about 2. If used ininverters, moreover, very short commutation times are achieved. Theprotective circuit arrangement can therefore be considered as acommutation aid, which allows one to go to high inverter outputfrequencies. Thus, operation of transistorized inverters of high outputfrequency and high output becomes possible.

Examples ofembodlments of this invention will be more fully explainedwith reference to the drawings.

DESCRIPTION OF THE DRAWINGS FIG. I is an electrical schematic diagram ofa protective circuit arrangement for an PNP auxiliary transistor.

FIG. 2 is an electrical schematic diagram ofa protective circuitarrangement for a pnp auxiliary transistor.

FIG. 3 is a graph of the voltage and current curves during the switchingoff of the switching transistor with time plotted along the horizontalaxis and voltage and current plotted along the vertical axis.

FIG. 4 is an electrical schematic diagram of an inverter with twoprotective circuits according to FIG. 1.

DETAILED DESCRIPTION FIG. II shows an NPN switching transistor T, whosecollector K is connected with a positive input terminal P and whoseemitter E is connected with a negative input terminal N through a loadwhich consists of an inductance L and an ohmic load resistor Ra. Insteadof a single switching transistor T, several switching transistors canalso be provided which are connected in series or in parallel. A DCsupply voltage U, is connected between the input terminals P and N. The

switching transistor T is fully turned on by a control signal at itsbase. Let us assume that the transistor current I which is the maximumpermissible for the transistor type in question flows through it as theload current la.

The collector-emitter path K-E of the switching transistor T is shuntedby a protective circuit arrangement which acts like a voltage-dependentresistor. It consists of a series connection of a resistor R with acontrolled auxiliary valve H and a control device for the auxiliaryvalve H. The resistor R must absorb the switching-off losses. It may bepurely ohmic or may be voltage-dependent. Its value must be equal to orsmaller than the value R U ll An NPN transistor is used as the auxiliaryvalve H in the embodiment shown in FIG. 1. The resistor R is connectedbetween the collector of the auxiliary transistor serving as valve H andthe positive input terminal P. If an NPN transistor is chosen as theauxiliary valve H, it is not possible to interchange the order of theauxiliary valve H and the resistor R shown in FIG. 1. The resistor ofthe series circuit being shunted across the collectoremitter path of theswitching transistor is connected to the collector of the switchingtransistor and the emitter of the NPN auxiliary transistor of the seriescircuit being shunted across the collector-emitter path of the switchingtransistor is connected to the emitter of the switching transistor. I

The auxiliary valve H poled in the conduction direction of the switchingtransistor T is controlled by an automatic control device, which isconnected between the control electrode B of the auxiliary valve I-I,i.e., to the electrode B of the auxiliary transistor, and the collectorK of the switching transistor. The control device is designed so that itsupplies a current into the control electrode B of the auxiliarytransistor H as a function of the voltage rise between the collector Kand the emitter E of the switching transistor T. Therefore, as long asthe switching transistor T carries a maximum load current I or a smallload current, the control device is not operative, and the auxiliarytransistor H is cut off.

As shown in FIG. l, the control device consists of a series circuit of acapacitor C and a charging resistor R. In many cases the resistance ofthe leads of the capacitor C may already be sufficient. In the circuitshown in FIG. 1, if an NPN auxiliary transistor is used, the seriescircuit is arranged between the positive input terminal P and thecontrol electrode B of the NPN auxiliary transistor H. This seriescircuit may also be shunted by a Zener diode N. The Zener voltage ofsaid Zener diode N is chosen so that the auxiliary transistor H is fullyturned on when the Zener voltage valve is applied to the controlelectrode of the auxiliary transistor H.

switching transistor T. The resistor R is located between the emitter Eof the NPN switching transistor T and the collector K of the PNPauxiliary transistor H. The series circuit of the capacitor C and thecharging resistor R is situated between the electrode B of the PNPauxiliary transistor H and the emitter E of the switching transistor T.Here too, a Zener diode n is provided.

FIG. 3 shows, in principle, the curves of the transistor voltage U andthe transistor current I plotted against the time it when the switchingtransistor T is switched off in a protective circuit such as shown inFIGS. 1 or 2.

Let us assume that up to the time t the transistor current I which,according to assumption, is the maximum permissible current for thistype, flows through the switching transistor T. This current has thesame magnitude as the load current la. The transistor voltage U betweenthe collector K and the emitter E of switching transistor T ispractically zero. At the time t, the control signal at the base of theswitching transistor T is now turned off. Without the protective circuitthe transistor voltage U would now jump to a voltage value U (notshown), which is given by the relation U L dI /dt, where L,, is thevalue of the inductance L and dI /dt is the rate of change of the loadand transistor current I The transistor current I would continue to flowfor some time up to t as indicated by the dashed line curve I shown inFIG. 3. The switching transistor T would have to absorb at each point intime t between t and t, a power loss P,,,,(t) which is obtained by theproduct P,,,,(t) U ,,(t) 1-,,(t). The energy U, converted into heat inthe switching transistor T between the time and t, is calculated byintegrating the power loss P,, (t).

If the protective circuit shown in FIGS. 1 or 2 is present, there is,after the switching transistor T is switched off at the time t a rise ofthetransistor voltage U which is large enough at the time, t, toovercome the threshold voltage of the base-emitter path of the auxiliarytransistor H. A displacement current flows from 1 on through thecharging resistor R and the capacitor C into the control electrode B ofthe auxiliary transistor H. The latter is thereby turned on increasinglyfrom the time t on. Thus, an increasing share of the load current 1,,can flow through the resistor R and the auxiliary valve H. Thetransistor current I consequently decreases more and more, as shown inFIG. 3, until it has reached the value 1 0 at the time t;,. With asuitable choice of the capacity of the capacitor C, a purely ohmicswitching-off behavior for the transistor current I and the transistorvoltage U shown in FIG. 3, can be obtained. A considerable portion ofthe magnetic energy stored in the inductance L prior to theswitching-off is converted into heat in the resistor R during the timeinterval t The energy converted into heat in the switching transistor Tis therefore considerably smaller than the stated value E For limitingthe voltage at the switching transistor T, the Zener diode N isprovided. It provides for the removal of all over-voltages occurring atthe switching transistor T, so that they do not lead to the destructionof the switching transistor T. Its Zener voltage U, is chosen so thatthe auxiliarytransistor H is fully turned on when the Zener voltagevalve is applied to the control electrode, i.e., the base B of theauxiliary transistor through saturation of the Zener diode. If, afterexceeding the supply voltage U,,, the transistor voltage U reaches theZener voltage U,, the auxiliary transistor H is turned on to such anextent that across it and the resistor R just the sum voltage U drops.During the switching-off process, the then still flowing load current1,, commutates from the switching transistor T to the series circuitconsisting of the resistor R and the auxiliary valve H and the currentin the switching transistor T becomes zero. The Zener voltage U, remainsat the switching transistor T only until the current in the auxiliarytransistor H also has become zero. Subsequently the DC supply voltage U,appears across the collectoremitter path K-E of the switching transistorT.

With saturation of the Zener diode n and turning on of the auxiliarytransistor H, the transistor voltage U is equal to the Zener voltage U,and is kept constant and the auxiliary transistor H goes through theentire field of characteristics from the conduction voltage in saturatedoperation up to the maximum voltage. Correspondingly, the voltage dropacross the resistor R decreases from the maximum voltage at fullcurrent.

It has been found also that in the event of occurring overvoltages, byfar the largest part of the available energy is transferred to theresistor R and that the switching transistor T is not exposed toexcessive stresses.

FIG. 4 shows an inverter W which is connected on the input side to apositive input terminal I and a negative input terminal N, and on theoutput side to an inductive three-phase load with the phase inputs R, S,T. The input terminals P and N may be fed, for example, by a battery ora rectifier with valves. The inductive load may be a rotating-fieldmachine, which may also, for example, be connected to the output of theinverter W through a transformer. The inverter W consists of sixswitching transistors T3, T4, T5, T6, T7, and T8 in a three-phase bridgecircuit. Each group of three switching transistors T3, T4, T5, and T6,T7, T8, is associated with a protective circuit arrangement S1 and S2,respectively, such as shown in FIG. 1.

The first protective circuit S1, consisting of the components H1, R1,Cl, r'l and n1, is provided for the group of three switching transistorsT3, T4 and T5 which are each connected directly or through a choke (notshown) to the positive input terminal P Correspondingly, the secondprotective circuit S2, consisting of the components H2, R2, C2, R'2 andn2, is provided for the group of three switching transistors T6, T7 andT8 which are connected to the negative input terminal N Components HIand H2 are auxiliary transistors. Components R1 and R2 are resistors.Components Cl and C2 are capacitors.'Components Rl and R'2 areresistors. Components nl and n2 are diodes.

In order to avoid short circuits, diodes n3, n4, n5, n6, n7 and n8 areprovided for each of the six switching transistors T3 to T8. The diodesn3 to n8 are each connected into a connecting line between therespective switching transistors T3 to T8 and the associated protectivecircuit arrangement S1 or S2. Each switching transistor T3, T4, T or T6,T7, T8 is therefore shunted by the series circuit of the associatedresistor R1 or R2, respectively, with the connected control device and adiode, n3, n4, n5, and n6, n7, n8. The diodes n3 to n8 are poled in theconduction direction of the auxiliary transistors Hi1 and H2,respectively, and in the flow direction of the corresponding switchingtransistor T3 to T8. For instance, upon switching off the NPN switchingtransistor T3, there is a current-carrying connection from the positiveinput terminal P through the resistor R1, the auxiliary transistor H1and the diode n3 to the phase input R. Upon switching off the switchingtransistor T3, this connection takes over a considerable part of theload current.

The protective circuits S1 and S2 of the inverter W, shown in FIG. 4,assure in all switching transistors T3 to T8 a switching-off curve ofthe transistor current and the voltage plotted against time whichcorresponds to that of a purely resistive load in the load circuit. Theswitching transistors T3 to T8 can, consequently, be stressed up to thethermal current limit and can, therefore, be utilized far better thancustomarily. Furthermore, the reverse voltage at the just disconnectingtransistor T3 to T8 can rise very fast to its final value, which isgiven by the Zener voltage of the Zener diode n1 or n2, respectively.This means that the commutation process from one switching transistor tothe next can take place considerably faster than with an inverter Wwithout protective circuits S1 and S2. As a result, higher inverteroutput frequencies can be achieved. Finally, the protective circuits S1and S2 assure that no excessive voltages occurring at the switchingtransistors T3 to T8 during switching off.

In the foregoing specification, the invention has been described inreference to specific exemplary embodiments. It will be evident,however, that variations and modifications in the embodiments explainedby way of illustration may be made without departing from the broaderscope and spirit of the invention as set forth in the appended claims.The specification and drawings are accordingly to be regarded in anillustrative rather than in a restrictive sense.

What is claimed is:

1. A protective circuit arrangement for a switching transistor, saidswitching transistor being arranged in an inductive load circuit forswitching the load current on and off, said protective circuitarrangement comprising:

a. a series circuit comprising a resistor and a controlled auxiliaryvalve, said series circuit being shunted across the collector-emitterpath of said switching transistor; and

b. a control device for controlling the control electrode of saidauxiliary valve as a function of the voltage rise at said switchingtransistor.

2. The protective circuit arrangement according to claim 1 wherein saidcontrolled auxiliary valve is a transistor.

3. The protective circuit arrangement according to claim 2 wherein saidcontrol device comprises a series circuit of a capacitor and a chargingresistor, said control device being connected between the controlelectrode of said auxiliary valve and the collector of said switchingtransistor.

4. The protective circuit arrangement according to claim 3 and furthercomprising a Zener diode, said Zener diode being shunted across saidseries circuit comprising said capacitor and said charging resistor, theZener voltage of said Zener diode being chosen so that said auxiliarytransistor is fully turned on when the Zener voltage value is applied tothe control electrode of said auxiliary transistor.

5. The protective circuit arrangement according to claim 1 wherein saidcontrolled auxiliary valve is an NPN transistor.

6. The protective circuit arrangement according to claim 5 wherein saidresistor of said series circuit being shunted across thecollector-emitter path of said.

switching transistor is connected to the collector of said switchingtransistor; and the emitter of said npn auxiliary transistor of saidseries circuit being shunted across the collector-emitter path of theswitching transistor is connected to the emitter of the switchingtransistor.

7. The protective circuit arrangement according to claim 1 wherein saidauxiliary valve is a PNP transistor.

8. The protective circuit arrangement according to claim 7 wherein saidresistor of said series circuit being shunted across thecollector-emitter path of said switching transistor is connected to theemitter of said switching transistor; and the emitter of said PNPauxilia'ry transistor is connected to the collector of said switchingtransistor.

9. A protective circuit arrangement adapted for use with an inverterhaving a plurality of switching transistors, said inverter beingconnected on the output side to an inductive load and on the input sideto a positive input terminal and to a negative input terminal, saidprotective circuit arrangement comprising:

a. a plurality of series circuits each comprising a resistor and acontrolled auxiliary valve, each said series circuit being shuntedacross the collectoremitter path of one or more of said switchingtransistors; and

b. a plurality of control devices for controlling the control electrodeof each said auxiliary valve'as a function of the voltage rise at therespective switching transistor. v i

10. The protective circuit arrangement according to claim 9 wherein saidplurality of switching transistors comprises six switching transistorsarranged in two groups, each group comprising three switchingtransistors, and said protective circuit arrangement comprises:

a. two said series circuits, each circuit comprising a resistor and acontrolled auxiliary valve, one said series circuit being associatedwith each said group of three switching transistors; and

b. two said control devices, one said control device for each of saidseries circuits.

11. The protective circuit arrangement according to claim 10 whereinsaid controlled auxiliary valves are transistors.

12. The protective circuit arrangement according to claim 10 whereinsaid control devices each comprise a series circuit of a capacitor and acharging resistor, each said control device being connected between thecontrol electrode of said auxiliary valve, of one of said seriescircuits and the collectors of one group of said three switchingtransistors.

14. The protective circuit according to claim 10 wherein one said groupof three switching transistors is connected to the positive inputterminal of said inverter and the other said group of three switchingtransistors is connected to the negative input terminal of saidinverter.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,729,655 Dated April 24, 1973 In n fl Werner Gratzke It is certifiedthat error appears in the above-identified patent and that saidvLettersPatent are, hereby corrected as shown below:

Column 4, lines 63 and 64, change "diode N" to read diode n-;

Column 5, line 39, change "time to read i Column 5, line 65, change"diode N" to read -diode n.

I Signed and sealed this 18th day o f'December- 1973.

(SEAL).

Attest:

EDWARD M.FLETCIIER,JR. v RENE D. TEG'IMEYI-IR Attesting" Officer ActingCommissioner of Patents 1 FORM P Q usc omm -oc scan-P69 fi u.'$.GOVERNMENT PRINTING-OFFICE): is}; o ded-s34,

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,729,655 Dated April 24, 1973 In n (s) Werner Gratzke It is certifiedthat error appears in the above-identified patent and that said'LettersPatent arev hereby corrected as shown below:

Column 4, lines 63 and 64, change "diode N" to read -diode n;

Column 5, line 39, change "time to read Column 5, line 65, change "diodeN" to read -diode n-.

Signed and sealed this' 18th day of December 1973.

(SEAL) Attest:

EDWARD M.FLETCIIER,JR. I RENE 1). TEG'IMEYI-IR Attesting Officer ActingCommissioner of Patents FORM Po-mso (10-69) 1 USC OMM-DC 60376-p59usJGOV'ERNHENT mm'rmo'orncl is p ied-S34,

1. A protective circuit arrangement for a switching transistor, saidswitching transistor being arranged in an inductive load circuit forswitching the load current on and off, said protective circuitarrangement comprising: a. a series circuit comprising a resistor and acontrolled auxiliary valve, said series circuit being shunted across thecollector-emitter path of said switching transistor; and b. a controldevice for controlling the control electrode of said auxiliary valve asa function of the voltage rise at said switching transistor.
 2. Theprotective circuit arrangement according to claim 1 wherein saidcontrolled auxiliary valve is a transistor.
 3. The protective circuitarrangement according to claim 2 wherein said control device comprises aseries circuit of a capacitor and a charging resistor, said controldevice being connected between the control electrode of said auxiliaryvalve and the collector of said switching transistor.
 4. The protectivecircuit arrangement according to claim 3 and further comprising a Zenerdiode, said Zener diode being shunted across said series circuitcomprising said capacitor and said charging resistor, the Zener voltageof said Zener diode being chosen so that said auxiliary transistor isfully turned on when the Zener voltage value is applied to the controlelectrode of said auxiliary transistor.
 5. The protective circuitarrangement according to claim 1 wherein said controlled auxiliary valveis an NPN transistor.
 6. The protective circuit arrangement according toclaim 5 wherein said resistor of said series circuit being shuntedacross the collector-emitter path of said switching transistor isconnected to the collector of said switching transistor; and the emitterof said npn auxiliary transistor of said series circuit being shuntedacross the collector-emitter path of the switching transistor isconnected to the emitter of the switching transistor.
 7. The protectivecircuit arrangement according to claim 1 wherein said auxiliary valve isa PNP transistor.
 8. The protective circuit arrangement according toclaim 7 wherein said resistor of said series circuit being shuntedacross the collector-emitter path of said switching transistor isconnected to the emitter of said switching transistor; and the emitterof said PNP auxiliary transistor is connected to the collector of saidswitching transistor.
 9. A protective circuit arrangement adapted foruse with an inverter having a plurality of switching transistors, saidinverter being connected on the output side to an inductive load and onthe input side to a positive input terminal and to a negative inputterminal, said protective circuit arrangement comprising: a. a pluralityof series circuits each comprising a resistor and a controlled auxiliaryvalve, each said series circuit being shunted across thecollector-Emitter path of one or more of said switching transistors; andb. a plurality of control devices for controlling the control electrodeof each said auxiliary valve as a function of the voltage rise at therespective switching transistor.
 10. The protective circuit arrangementaccording to claim 9 wherein said plurality of switching transistorscomprises six switching transistors arranged in two groups, each groupcomprising three switching transistors, and said protective circuitarrangement comprises: a. two said series circuits, each circuitcomprising a resistor and a controlled auxiliary valve, one said seriescircuit being associated with each said group of three switchingtransistors; and b. two said control devices, one said control devicefor each of said series circuits.
 11. The protective circuit arrangementaccording to claim 10 wherein said controlled auxiliary valves aretransistors.
 12. The protective circuit arrangement according to claim10 wherein said control devices each comprise a series circuit of acapacitor and a charging resistor, each said control device beingconnected between the control electrode of said auxiliary valve of oneof said series circuits and the collectors of one group of said threeswitching transistors.
 13. The protective circuit arrangement accordingto claim 10 and further comprising six diodes, one diode for each of thesix switching transistors, each diode being connected between itsrespective switching transistor and the associated series circuit ofsaid protective circuit arrangement, and each diode being poled in thedirection of said auxiliary valve of said associated series circuit. 14.The protective circuit according to claim 10 wherein one said group ofthree switching transistors is connected to the positive input terminalof said inverter and the other said group of three switching transistorsis connected to the negative input terminal of said inverter.